Methods of randomly dispersing chopped carbon fiber and consolidating the materials together to make a fiber reinforcement mat

ABSTRACT

Methods of randomly dispersing chopped carbon fiber and consolidating the materials together to make a fiber reinforcement mat are disclosed. The term “Z-mat” is used to describe the claimed random fiber mat in which the chopped fibers are not preferentially oriented in either the x or y direction in the in-plane directions of the mat. The stiffness in the Z-mat is substantially independent of the test direction in the plane of the mat. The Z-mat offers the distinct advantages of higher stiffness and lighter weight.

RELATED APPLICATIONS

[0001] This application claims priority from Provisional ApplicationSerial No. 60/307,599, filed Jul. 26, 2001, entitled “Methods ofRandomly Dispersing Chopped Carbon Fiber and Consolidating the MaterialsTogether to Make a Fiber Reinforcement Mat,” the entire disclosure ofwhich is hereby incorporated herein by reference.

FIELD OF INVENTION

[0002] This invention relates to an apparatus and method for randomfiber mats. The invention has particular applicability in chopping acarbon tow into strands of shorter lengths and distributing the strandsin random orientation.

BACKGROUND

[0003] Various methods have been tried to chop, randomize, andconsolidate carbon fiber in random mat format. The primary problems aremaximizing randomness of fiber orientation, minimizing variability ofmat area weight, and making a product that is easily handled, all ofwhich cause problems with producing an effective reinforcement mat.

SUMMARY OF THE INVENTION

[0004] By combining different fiber spreading techniques with severaldesigns of air flow in an enclosed chute a number of trials have beenmade to identify the right combination of fiber spread, air flow, andchute geometry to randomly orient carbon fiber chop while maintaininguniform area weight distribution. In addition, several trials have beenmade with and without various backing materials to improve the overallhandling characteristics of the resulting mat.

[0005] In one embodiment, three 48K tows have been successfully choppedand randomly distributed while maintaining a very uniform area weightdistribution (up to 230 g/m²). A powder binder was added and theresulting materials consolidated under heat and pressure to make a 16″wide mat at 23 feet per minute. These values are typical of commercialfiber production. Fiberglass veils have also been added to both sides ofthe mat to make a hybrid as well.

[0006] Different embodiments of this invention include the following:(1) An equipment that provides a dry, mechanical way to assemble fibersinto mat form. (2) A mat in which the randomness of chopped carbon fiberin the mat is maximized while minimizing the variability of the areaweight in the product by using air, a distribution bar, and a specificchute design. (3) A mat in which the chopped carbon fiber can beconsolidated with fiberglass or polyester veil or scrim to make a fiberhybrid mat. (4) The mat of embodiments (2) and (3) that is easilyhandled without susceptibility to tear. (5) The mat of embodiments (2)and (3) that is easily wet out providing excellent resin penetration forcomposite part manufacturing. (6) A glass veil/carbon fiber mat that hasa very smooth surface with almost no protruding fibers. (7) Methods ofcarrying out the embodiments of (1)-(6) above.

[0007] This invention can be used commercially to chop, distribute, andbind carbon fiber with fiberglass veil to make reinforcement mat forcomposites. It may be incorporated into the carbon fiber production lineas an add-on to the sizing section of the line. Or, it may be set up asa separate off line entity to chop, distribute, and bind alreadyproduced carbon fiber tows. There are a variety of reasons to use carbonfiber mat; the most important of which is to maximize mechanicalproperties in all directions. The unique advantage provided by usingfiberglass veil as a carrier for the carbon fiber is that it willimprove surface finish in products such as automobile and boat paneling.

[0008] As will be realized, this invention is capable of other anddifferent embodiments, and its details are capable of modifications invarious obvious respects, all without departing from this invention.Accordingly, the drawings and description are to be regarded asillustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a view of the process flow diagram of the process formaking the chopped fiber mat of this invention.

DETAILED DESCRIPTION

[0010] This invention embodies a chopper system that cuts carbon fibertow (up to 320 K) into shorter lengths (up to 3″) and distributes themin random orientation. It also includes a powder-coater and laminatorthat consolidates the materials to make a randomly orientedreinforcement mat that minimizes area weight variability and hasexcellent handling characteristics.

[0011] The invention also includes resulting carbon fiber mats that canbe made with the equipment listed, including but not limited to thefollowing:

[0012] (1) Mat made with chopped carbon fiber up to 3″ in length boundtogether with a powder resin and without any backing material.

[0013] (2) Mat made as in item 1 that includes a backing material suchas a fiber (i.e. polyester, glass, or the like) veil or scrim or sometype of release paper.

[0014] The methods of this invention include some or all of thefollowing steps:

[0015] (1) The chopper chute and air distribution equipment used to laythe carbon fiber on the veil is unique to controlling the area weightdistribution and random fiber orientation.

[0016] (2) The spreading process used to open the carbon fiber tow priorto chopping is unique to helping randomize the fiber distribution.

[0017] (3) The consolidation of fiberglass veil and chopped carbon fiberwith a powder coating improves the handling, appearance, and mechanicalproperties of the mat.

[0018] In one embodiment, this invention uses 95% carbon fiber tow fromPAN (polyacrylonitrile) made by a continuous carbonization process. Thismaterial is then converted into various product forms to the processingof the material. The “Z-mat” process chops the carbon fiber into 1″, 2″,or 3″ lengths and randomly disperses and binds them in mat form. Theterm “Z-mat” is used to describe the claimed random fiber mat in whichthe chopped fibers are not preferentially oriented in either the x or ydirection in the in-plane directions of the mat. The Z-mat offers thedistinct advantages of higher stiffness and lighter weight.

[0019] The maximum line speed is dependent on the desired area weight ofthe end product. Essentially the feed rate of carbon fiber, number oftows, and the conveyor speed dictate the resulting area weight. In oneembodiment, the line was designed to process a maximum of 150 lb/hrcarbon fiber at most all product area weights.

[0020] The details of the process, with reference to FIG. 1, areexplained below.

[0021] (1) Raw Material Handling:

[0022] (a) Carbon Fiber:

[0023] The feed creel is a steel frame with 36 evenly spaced spoolpositions. Carbon fiber spools are mounted on each spool position. Thecreel aligns the fibers as parallel as possible and applies tension asnecessary to provide a uniform distribution of fiber to the rest of theprocess. The required number of ends from the carbon fiber spools isthreaded through the guide bars at evenly spaced intervals. Each spoolposition utilizes a friction brake to sustain adequate tension on thetow as it passes on to the process. Changing the dead weight on thebrake varies tension.

[0024] (b) Backing/Support:

[0025] Two unwinders are positioned on the line to allow backingmaterial, preferably, up to 64″ wide to be applied to the mat. Backingcan be fiberglass veil, fiberglass scrim, release paper or the like. Oneunwinder is located just after the feed creel at the start of theconveyor belt. This unwinder is used to feed backing to the process thatwill catch the fiber as it falls from the chopper. The second unwinderis mounted after the chopper and on top of the laminator. This unwinderis used to apply material to the top of the mat just before it entersthe laminator. Since the backing will be, preferably, up to 62″ wide andcan weigh up to 100 lbs., a platform and walkway were provided to allowtwo people access to this unit.

[0026] Both unwinders control tension using load cells and variablefrequency drive motors. The operator will be able to set tension and thesystem will control it to a minimum +10% of set point. Both unwindersare modular and can be removed when not in use.

[0027] (2) Spreading Process:

[0028] After the creel the carbon fiber is threaded through a series ofbars that help spread the fiber. The first bar is a guide bar tomaintain tow position coming off the creel. The next bars spread eachtow up to 1 ⅝″ wide. The final bar helps feed the spread fiber to thechopper nip. It is important that fiber alignment be maintained throughthis process.

[0029] Water mist can be applied to the spread tow just before it leavesthe spreading system. The mist is used to keep fly down in the choppingprocess. However, it does cause the spread tow to narrow a bit as aresult of wicking.

[0030] (3) Chopping Process:

[0031] A rotary chopper was used to cut carbon fiber tow into lengths of1″, 2″, or 3″. The length is adjustable by changing the blade positionson the chopper head. The chopped pieces fall down through an enclosedchute to the conveyor belt where it is taken to the rest of the process.Immediately after the chopper head is a randomizer. The randomizerconsists of a stationary roll with several pins extending outward. It isused to break up the fiber bundles as they fall to the conveyor. Also atthe top of the chopper is a cot cleaner. This device is a series of airtubes directed at the cot roller that oscillate back and forth.High-pressure air is passed through each tube to help knock off anyfiber collecting on the cot. The sides of the chute are adjustable toallow the operator to vary product width up to 60″. The chopping andspreading equipment are modular and can be removed when not being used.

[0032] The chute can be made to run under negative pressure by insertingan air laying plate at the bottom. This perforated plate is insertedover top of the conveyor and under the porous backing material. Theplate is connected to a filter and fan and draws the chopped fiber downto the backing material. This plate can preferably be used with sometype of porous backing passing over it. Without the backing carbon fiberwill stick to the plate and not be moved to the rest of the process.

[0033] (4) Coating:

[0034] Preferably, dry coatings could be used with the Z-mat process. Apowder coater located just after the chopper module applies thesecoatings. The powder coater is a large hopper with a rotating roller inthe bottom opening. The surface of the roller is roughened enough topick the powder up from inside the hopper. Outside the hopper, a brushoscillates back and forth on the surface of the roller to knock thepowder off. The loosened powder falls through a chute and onto the mat.Adjusting the speed of the roller controls the powder application rate.A level controller is supplied in the hopper that will indicate when itneeds to be filled. A platform/walkway is provided around the unit tohelp with maintenance and material change out. The powder coater ismodular so it can be removed when not used.

[0035] (5) Laminating Process:

[0036] The laminator consists of two Teflon belts on top of each otherpressed together by a series of nips. A radiant panel to preheat the matis mounted just prior to the entrance of the laminator. The preheathelps to avoid flashing the coatings which would cause poor adhesion tothe fiber. The first half of the laminator is heated up to a maximum425° F. by elements on top and bottom of the belts. At this point thepowder coating and/or the coating on the backing material is melted andflows into the carbon fiber. The second half of the laminator is cooledto 130° F. by coils on top and bottom of the belts that circulate waterfrom a 10-ton chiller. This is where the coating(s) harden back to theirdry form, thus freezing the fibers together in mat form. The 10-tonchiller will be mounted outside, adjacent to the line. Pressure,temperature and residence time determine how well the mat is fusedtogether. The belt speed and type of powder coating and/or type ofcoating on the backing material dictate the laminator temperature setpoints. Four-limit switches and pneumatic controls monitor the edges ofthe mat to make sure the product doesn't walk more than +1 inch. Theopening through the laminator can be manually adjusted from 0-1″ widedepending on the desired product thickness. The product is pulled off ofthe belts as it exits the laminator. Any material left sticking to thebelt is wiped off by circulating cleaning rolls. The material knockedoff the belts is collected in receptacle trays that must be cleaned outroutinely.

[0037] (6) Winding:

[0038] The winder spools the product exiting the laminator undercontrolled tension. Load cells and a variable frequency drive motorcould be used to control tension. The operator sets the desired tensionon the product. The product can be spooled up to 18″ diameter. The finalwidth of the product is determined by how wide the operator sets theedge cutters. The edge cutters are designed to remove excess materialfrom the product as necessary. When the desired length is reached theoperator must manually cut the material. The final product is placed ona cart for testing, if required, and packaging.

[0039] (7) Electrical & Controls:

[0040] All equipment, except the chiller, would preferably be controlledat the man-machine interface (MMI) mounted on the feed creel. The MMIwill have all necessary settings fully adjustable by the operator.Certain functions associated with maintenance and engineering shouldpreferably be accessible by key switch. The chiller should preferablyhave its own control panel mounted on one of the electrical controlcabinets next to the end of the Z-mat line. Receptacles will be locatedabove the line to provide power to all modular equipment. All otherequipment could be hardwired.

[0041] Samples made by the Z-mat Process

[0042] (1) PANEX® 33 ZM

[0043] PANEX® 33 ZM is a chopped carbon fiber mat manufactured fromPANEX® 33 continuous tow that is made from polyacrylonitrile (PAN)precursor. PANEX® 33 ZM chopped carbon fibers are available with avariety of sizing and/or binder formats for different compositeprocessing methods and for compatibility with a wide range of standardresin systems. TABLE 1 Typical Fiber Properties of PANEX ® 33 ZM U.S.Units SI Units Tensile Strength* 550 Ksi 3800 Mpa Tensile Modulus* 33Msi 228 GPa Electrical Resistivity 0.00068 ohm-in 0.00172 ohm- cmDensity 0.065 lb/in³ 1.81 g/cc Fiber Diameter 0.283 mils 7.2 micronsCarbon Content 95% 95%

[0044] TABLE 2 Typical Laminate Properties of PANEX ® 33 ZM¹ (At 40% w/wcarbon content) Tensile Tensile Modulus: Flexural Flexural Strength:×10⁶ psi Test Strength: Modulus: Test Resin Type ksi(MPa) (GPa) Methodksi(MPa) ksi(MPa) Method Polyester* 23.4 (161) 2.45 (16.9) ASTM 38.6(266) 2.43 (16.8) ASTM D638 D790 Vinyl Ester** 25.7 (177) 2.51 (17.3)ASTM 37.2 (256) 2.61 (24.9) ASTM D638 D790 Polyurethane*** 27.2 (188)3.22 (22.2) ASTM 32.6 (225) 2.43 (16.8) ASTM D638 D790

[0045] The chopped fiber mat PANEX® 33 ZM has several variations asshown in the accompanying selector chart for sizing format, resincompatibility, and packaging information.

[0046] PANEX® 33 ZM chopped carbon fiber mat was made with ourcopolyester sizing on the fibers and a polyester powder binder holdingthe mat together. PANEX® 33 ZM products are defined by our nine-digitproduct code which is detailed below. Nine Digit Format Code 1^(st)Digit 2^(nd) & 3^(rd) Digits 4^(th) Digit 5^(th) Digit (Backing) (MatWidth) (CF Length) (CF Area Weight) O-None 00-Custom 1-1″ 0-CustomC-Custom 12-12 2-2″ 1-100 g/m² V-Fiberglase Veil 24-24″ 3-3″ 2-150 g/m²T-Thermoplastic Veil 36-36″ X-Custom 3-200 g/m² P-Thermoplastic Scrim48-48″ 4-250 g/m² S-Fiberglass Scrim 60-60″ 5-300 g/m² F-ThermoplasticFilm 6^(th) Digit (Sizing Type) 7^(th) Digit (% Sizing) 0-None 0 0.01-Epoxy 1 0.55 ± 0.25 2-Polyester 2  1.0 ± 0.25 3-Polyester (StyreneSoluble) 3  1.5 ± 0.25 4-Polypropylene 4 2.0 ± 0.5 5 3.0 ± 0.5 6 4.0 ±1.0 7 6.0 ± 1.0 8 8.0 ± 1.0 X-Custom 9 >9.0 8^(th) Digit (Binder Type)9^(th) Digit (% Binder) 0-None 0 0.0 1-Epoxy Powder 1 1.0 ± 0.52-Polyester Powder 2 2.0 ± 0.5 3-Polyester Powder (Styrene 3 3.0 ± 0.5Soluble) 4-Thermoplastic Fiber 4 4.0 ± 0.5 5 6.0 ± 1.0 6 8.0 ± 1.0 7 9.0± 1.0 8 >10.0 X - Custom

[0047] (2) PANEX® 33 ZM-V

[0048] PANEX® 33 ZM-V is a chopped carbon fiber mat manufactured fromPANEX® 33 continuous tow that is made from polyacrylonitrile (PAN)precursor. PANEX® 33 carbon fibers are available with a variety ofsizing and/or binder formats for different composite processing methodsand for compatibility with a wide range of standard resin systems. ZM-Vis a randomly oriented chopped strand mat product composed of specifiedlengths (1″, 2″ or 3″) of carbon fiber sandwiched between layers ofpolymer or fiberglass veil. TABLE 2 Typical Fiber Properties of PANEX ®33 ZM U.S. Units SI Units Tensile Strength* 550 Ksi 3800 Mpa TensileModulus* 33 Msi 228 GPa Electrical Resistivity 0.00068 ohm-in 0.00172ohm-cm Density 0.065 lb/in³ 1.81 g/cc Fiber Diameter 0.283 mils 7.2microns Carbon Content 95% 95%

[0049] TABLE 4 Typical Laminate Properties of PANEX ® 33 ZM-V (At 40%w/w carbon content) Tensile Tensile Modulus: Flexural Flexural Strength:×10⁶ psi Test Strength: Modulus: Test Resin Type ksi(MPa) (GPa) Methodksi(MPa) ksi(MPa) Method Polyester* 24.1 (166) 2.48 (17.1) ASTM 39.2(270) 2.40 (16.5) ASTM D638 D790 Vinyl Ester** 24.6 (170) 2.42 (16.7)ASTM 35.6 (245) 2.65 (18.3) ASTM D638 D790 Polyurethane*** 25.2 (174)2.81 (19.4) ASTM 30.4 (210) 2.39 (16.5) ASTM D638 D790

[0050] The chopped fiber mat PANEX® 33 ZM-V has several variations asshown in the accompanying selector chart for sizing format, resincompatibility, and packaging information. PANEX® 33 ZM-V chopped carbonfiber mat was made using copolyester sizing on the fibers and apolyester binder holding the mat together. PANEX® 33 ZM-V products canbe made in many different widths and fiber area weights. PANEX® 33 ZM-Vwas made in product weights of 100, 150, 200, 250 and 300 g/m² andwidths of 61 cm (24 in), 91 cm (36 in), 122 cm (48 in), and 152 cm (60in). The process of this invention also makes other widths and areaweights. The mat binder and carbon fiber sizing used are as follows: MatBinders Wt. % Binder Carbon Fiber Sizing Wt. % Sizing Epoxy 3-7 Epoxy0.55-2 Polyester 3-7 Polyester 0.55-2 Polypropylene 3-7 Polypropylene0.55-2 Polymer Fibers 3-7 Copolyester 0.55-2

[0051] Typically, the polyester is soluble styrene. The different typesof backing used are: (1) Fiberglass Veil; (2) Thermoplastic Veil and (3)Thermoplastic Film.

[0052] The above description is presented to enable a person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe preferred embodiments will be readily apparent to those skilled inthe art, and the generic principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the invention. Thus, this invention is not intended to belimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

[0053] This application discloses several numerical range limitations.Persons skilled in the art would recognize that the numerical rangesdisclosed inherently support any range within the disclosed numericalranges even though a precise range limitation is not stated verbatim inthe specification because this invention can be practiced throughout thedisclosed numerical ranges. A holding to the contrary would “let formtriumph over substance” and allow the written description requirement toeviscerate claims that might be narrowed during prosecution simplybecause the applicants broadly disclose in this application but thenmight narrow their claims during prosecution. Finally, the entiredisclosure of the patents and publications referred in this applicationare hereby incorporated herein by reference.

1. An apparatus for producing a chopped fiber mat, comprising a towspreader for spreading a fiber tow prior to a chopper that chops saidfiber tow into chopped fiber bundles, and a randomizer that breaks upthe chopped fiber bundles to produce fibers having a random orientationin a direction in the plane of the chopped fiber mat.
 2. The apparatusof claim 1, wherein the chopped fiber mat has a stiffness that issubstantially independent of a test direction in the plane of thechopped fiber mat.
 3. The apparatus of claim 2, wherein the choppedfiber mat has a ratio of a maximum-in-plane stiffness to aminimum-in-plane stiffness of 1.3.
 4. The apparatus of claim 2, whereinthe chopped fiber mat has a ratio of a maximum-in-plane stiffness to aminimum-in-plane stiffness of 1.2.
 5. The apparatus of claim 2, whereinthe chopped fiber mat has a ratio of a maximum-in-plane stiffness to aminimum-in-plane stiffness of 1.1.
 6. The apparatus of claim 1, whereinthe fiber tow is a carbon fiber tow.
 7. The apparatus of claim 1,wherein the chopped fiber mat further comprises a backing.
 8. Theapparatus of claim 7, wherein the backing is selected from the groupconsisting of a fiberglass veil, a thermoplastic veil, a thermoplasticscrim, a fiberglass scrim, a thermoplastic film, and combinationsthereof.
 9. The apparatus of claim 1, wherein the fiber tow comprises asizing and the chopped fiber mat comprises a powder binder.
 10. Theapparatus of claim 8, wherein the sizing is a polyester-containingsizing and the powder binder is a polyester powder binder.
 11. A methodfor producing a chopped fiber mat, comprising spreading a fiber tow,chopping the fiber tow to produce chopped fiber bundles and breaking upthe chopped fiber bundles to produce fibers having a random orientationin a direction in the plane of the chopped fiber mat.
 12. The method ofclaim 11, wherein the chopped fiber mat has a stiffness that issubstantially independent of a test direction in the plane of thechopped fiber mat.
 13. The method of claim 12, wherein the chopped fibermat has a ratio of a maximum-in-plane stiffness to a minimum-in-planestiffness of 1.3.
 14. The method of claim 12, wherein the chopped fibermat has a ratio of a maximum-in-plane stiffness to a minimum-in-planestiffness of 1.2.
 15. The method of claim 12, wherein the chopped fibermat has a ratio of a maximum-in-plane stiffness to a minimum-in-planestiffness of 1.1.
 16. The method of claim 11, wherein the fiber tow is acarbon fiber tow.
 17. The method of claim 11, wherein the chopped fibermat further comprises a backing.
 18. The method of claim 17, wherein thebacking is selected from the group consisting of a fiberglass veil, athermoplastic veil, a thermoplastic scrim, a fiberglass scrim, athermoplastic film, and combinations thereof.
 19. The method of claim11, wherein the fiber tow comprises a sizing and the chopped fiber matcomprises a binder.
 20. The method of claim 19, wherein the sizing isselected from the group consisting of a thermosetting sizing, athermoplastic sizing, and combinations thereof.
 21. The method of claim20, wherein the thermosetting sizing is selected from the groupconsisting of an epoxy-containing sizing and a polyester-containingsizing.
 22. The method of claim 20, wherein the thermoplastic sizing isselected from the group consisting of a polypropylene-containing sizingand a polyethylene-containing sizing.
 23. The method of claim 19,wherein the binder is selected from the group consisting of athermosetting binder, a thermoplastic binder, and combinations thereof.24. The method of claim 23, wherein the thermosetting binder is selectedfrom the group consisting of an epoxy-containing binder and apolyester-containing binder.
 25. The method of claim 23, wherein thethermoplastic binder is selected from the group consisting of apolypropylene-containing binder and a polyethylene-containing binder.26. The method of claim 23, wherein the thermoplastic binder is selectedfrom the group consisting of a powder-containing binder and afiber-containing binder.